Developing Drawing Strategies: Part 1 - AutoCAD 2011 and AutoCAD LT 2011: No Experience Required - page 116

FiGuRE 4.22 Using the Zoom Window option: positioning the rectangle
(top) and the new view after the ZOOM command (bottom)
3. From the Home tab ➢ Modify panel, click the Extend button.
In the command window, notice the second and third lines of text.
You're being prompted to select boundary edges (objects to use as
limits for the lines you want to extend/trim).